1. Field of the Invention
The present invention generally relates to a method for driving a photo-sensor. More specifically, the present invention has been directed to a driving method for a image sensor typically employed in a facsimile machine and an image scanner.
2. Description of the Related Art
Recently, a great attention is paid to a contact type image sensor capable of optically reading image information of an original at equal magnification as image input units of facsimiles and image scanners and other appliances. This contact type image sensor owns such particular advantages as compactness and low cost, since this image sensor is so arranged that the focussing optical system is eliminated by setting the image sensor directly to the original with keeping the contact condition, or another optical system having the shorter optical length of the focussing system.
In general, a contact type image sensor is fabricated such that a large number of photo-sensors having a width substantial equal to that of an original are arranged at a higher density so as to obtain a desirable resolution.
Then, a so-called "matrix driving type image sensor" is commercially available in which a plurality of photo-sensors are electrically connected in a matrix form so as to make such a matrix driving type image sensor compact at a lower cost by reducing a total number of the driving (or selecting) elements for scanning these photo-sensors.
In such a image sensor, a high-speed reading operation of information is demanded. To this end, a large quantity of photo-sensors must satisfy the following two conditions; 1) a better photo-responsive characteristic of a signal current, capable of responding incident light, and 2) a better voltage response characteristic of the signal current, capable of responding a drive voltage.
To achieve the improved photo-responsive characteristic, as one of the typical demanded characteristics, one conventional image sensor has been proposed in Japanese KOKAI (Disclosure) patent No. 58-18978 opened on Feb. 3, 1983. In this image sensor, two main electrodes are formed over one major surface of the amorphous semiconductor layer in order to detect the changes in the conductivity in response to the intensity of the incident light. Further, the auxiliary electrode is formed via the insulating layer on the other major surface. Then, the potential is applied to the amorphous semiconductor layer with utilizing two main electrodes and the auxiliary electrode.
Also, other conventional photo-sensors (image sensors) have been described in, for instance, Japanese KOKAI (Disclosure) patent applications No. 63-1055 opened on Jan. 6, 1988, and No. 60-239072 opened on Nov. 27, 1985. These photo-sensors each has the auxiliary electrode and the bias voltages applied to this auxiliary electrode are changed during the read time period and also non-read time period, whereby the photo-responsive characteristic thereof can be furthermore improved.
The typical structure and the connecting relationship between the structure of the conventional image sensor and the drive power source will now be summarized.
The structure of the conventional image sensor is so fabricated as follows. The auxiliary electrode and insulating layer are successively stacked on the insulating substrate. The amorphous semiconductor layer, such as an amorphous silicon (a-Si:H), is formed on the stacked layer. Further, a pair of main electrode are fabricated via the doped semiconductor layer for the ohmic contact on this amorphous semiconductor layer. The region between these main electrodes functions as the light receiving window.
In the above-described conventional image sensor described in Japanese KOKAI patent application No. 63-1055, in case that, for instance the amorphous semiconductor layer is of n type (including an intrinsic type semiconductor layer) and the doped semiconductor layer is of n.sup.+ type (including an n type), the high drive voltage is applied to one main electrode with respect to the potential of the other main electrode as the reference potential, the first bias voltage "-V.sub.1 " having the low potential is applied to the auxiliary electrode during the read time period and the second bias voltage "-V.sub.2 " (.vertline.V.sub.1 .vertline.&lt;.vertline.V.sub.2 .vertline.) is applied via the switch to the auxiliary electrode during the non-read time period. In this conventional photo sensor, the major carrier is an electron, one main electrode is a source, and the other main electrode is a drain. Although a detailed operation of this image sensor is described in the above patent application, a simple explanation will now be made. When the light is incident via the light receiving window upon the amorphous semiconductor layer, both the electrons and holes are produced in the amorphous semiconductor layer at a predetermined concentration corresponding to the quantity of the incident light. As a result, the conductivity of the amorphous semiconductor layer is increased. As a result, under this condition, if the drive voltage is applied between this pair of main electrodes, the signal current corresponding to the changes in this conductivity can be derived.
However, the electrons and/or holes remain in the amorphous semiconductor layer even when no light is incident upon the image sensor, so that the residual current continuously flows through the image sensor. This residual current may cause the photo-responsive characteristic to be deteriorated.
As clearly described in the above-described patent application No. 58-18978, a little improvement can be achieved in the above-described delay of the photo-responsive velocity of the conventional image sensor by applying the first bias voltage "-V.sub.1 " to the auxiliary electrode so as to bring the localized states within the amorphous semiconductor layer to the inert condition. Furthermore, as previously described, the second bias voltage "-V.sub.2 " is applied to the auxiliary electrode during the non-read time period in order to lower the concentration of the holes and electrons present in the amorphous semiconductor layer. As a result, when the electrons and holes produced by the above incident light are further recombined with each other, the time required during which the residual current reaches the originally set dark current level can be shortened so that the photo-responsive velocity (characteristic) may be furthermore improved.
As previously described in detail, the photo-responsive characteristic of the conventional image sensor can be eventually improved by way of the above-described sensor driving method. However, the remaining major characteristic, i.e., the voltage response characteristic of the conventional image sensor having no auxiliary electrode is not improved, but also deteriorated as compared with that of another conventional image sensor having the auxiliary electrode. This is because the carrier injection required for the quick voltage response operation of the conventional image sensor might be not properly carried out.
In particular, such a delay voltage response may cause serious problems in the matrix driving type image sensor. That is, in the matrix driving type image sensor, the pulsatory drive voltages are applied to the respective image sensor elements, as previously described.
In this case, when the signal current responses to the respective image sensors with respect to such drive pulses are delayed, the time period required for 1-line read operation becomes extremely long while the image sensor elements are sequentially selected so as to read the signals in the serial form. The high-speed signal read operation by the conventional image sensor can be hardly expected. For instance, in case that such a conventional linear image sensor is employed in the GIII type facsimile machine, since the total number of the sensor elements amounts to approximately 2,000 and the 1-line read time requires 5 to 10 milliseconds, the necessary voltage response time thereof is about 2.5 to 5 microseconds. However, as previously described in detail, the practical voltage response of the conventional image sensor in response to the above-described conventional drive voltage is on the order of milliseconds, namely very slow, which is not practically available.